Polycystic kidney disease (PKD) is the most common inherited disorder leading to chronic kidney disease and renal failure. Human pedigrees and rodent models have led to cloning of genes linked to PKD. Characterization of two mutant mouse strains, kat and kat2J, both of which develop progressive PKD, has linked Nek1 (Aspergillus NimA-related kinase) to the pathogenesis of PKD. To date, we have shown that Nek1 is involved early in DNA damage response and that cells without functional Nek1 are sensitive to ionizing radiation. Here, we propose that Nek1 is involved fundamentally in the pathogenesis of several forms of PKD, including human autosomal dominant PKD, as a factor that modifies disease progression in the setting of superimposed ischemic or genotoxic stress. Renal tubular cells in polycystic kidneys suffer from aberrant DNA damage repair responses, which result in both cyst- initiating, second-hit mutations to genes like PKD1 and PKD2, if the damage is passed on to proliferating daughter cells. They also undergo aberrant apoptosis in excessive numbers when the DNA damage is severe and irreparable. In this proposal, we will explore the roles of Nek1 protein kinase in ischemic injury, DNA damage, and apoptosis. In Aim 1, we will examine the expression of Nek1 in response to ischemia-reperfusion tubular injury, the effects of this injury on the progression of PKD in kat2J mice, and mutation frequency and spectrum induced by ischemic-oxidative injury in kat2J mice. In Aim 2, we will explore the roles of Nek1 in the DNA damage response and DNA repair. We will systematically determine how Nek1 is involved in sensing and responding DNA damage, by carefully dissecting potential upstream regulators and downstream effectors of Nek1, and by identification of key potential substrates of Nek1 involved in the DNA damage/repair pathway. In Aim 3, we will examine the detailed mechanisms by which Nek1 affects renal tubular cell apoptosis after injury, based on our evidence that Nek1 is crucial for regulating the VDAC component of the mitochondrial transition pore channel, which initiates the apoptotic caspase cascade when open. Our studies will provide novel insight into the molecular pathogenesis of PKD. They will help explain why cysts progress at different rates and how therapies to slow the progression of PKD can be designed rationally.